JSFiddle - React, Tailwind, and code Playground

JavaScript

var users = [{"name":"John","group":"t1"}];

var new_user;
for(var i=0; i<users.length; i++) {
  if(users[i].group === 't1') {
    new_user = users.slice(i);
    break;
  }
}

users.push(new_user);

var l = users.length - 1; 
users[l].name = 'Jack';

alert(users[0].name);
alert(users[1].name);